In general, it is widely conducted to embed glass fibers, chemical fibers or the like as reinforcing fibers in a rubber in order to improve strength and durability of rubber products such as rubber belts or rubber tires.
However, the reinforcing fibers themselves have poor compatibility with rubbers, resulting in poor adhesive property with rubbers. Therefore, if the reinforcing fibers are embedded in rubbers without treating the surface of the reinforcing fibers, the reinforcing fibers and the rubbers are not adhered or adhesive force therebetween is weak, so that they easily separate each other during use. Under such a state, the reinforcing fibers cannot satisfy performances required as a reinforcing material of rubber products.
For the above reason, where various reinforcing fibers are used in rubber products, it is generally and widely conducted to form on the surface of the reinforcing fibers a coating layer which improves a compatibility with a rubber used and therefore increases adhesive property therewith.
For example, JP-A-63-270877 (the term "JP-A" used herein means an "unexamined and published Japanese patent application") discloses a glass fiber cord having a coating which is formed by coating a mixed treating material of a resorcin/formalin condensate and H-NBR latex on glass fibers, and drying and curing the same. Further, JP-B-5-71710 (the term "JP-B" used herein means an "examined and published Japanese patent publication") proposes a fiber for reinforcing a rubber, further having a second layer formed by coating a halogen-containing polymer and an isocyanate on the above-described coating layer of the mixed treating material, and a third layer containing the same rubber as a matrix rubber formed on the second layer. However, the former cannot obtain a sufficient adhesive strength with a mixed rubber of N-NBR and H-NBR/ZDMA, and the latter requires coating a third layer, so that production steps are complicated and production cost increases.
As described above, it is generally and widely conducted to form a coating layer on the surface of reinforcing fibers in order to improve adhesive property between rubbers used and the reinforcing fibers. However, there are a variety in kinds of rubbers and reinforcing fibers recently used, and if the kinds of rubbers and reinforcing fibers used vary, the kind of the coating layer for improving adhesive property with those naturally varies.
In recent years, H-NBR/ZDMA rubbers are noted in various fields. A rubber obtained by mixing H-NBR/ZDMA rubber with H-NBR rubber has high heat resistance as compared with H-NBR rubber conventionally used. Therefore, such a mixed rubber can be used in the field in which the conventional rubbers could not been used, or can be used as a high performance-substitute for conventional rubbers.